A hybrid power train using a planetary gear set and two motor generators together with en engine can achieve the function of a continuously variable transmission, which is electrically operated, without a transmission, by controlling the speed of the motor generators. Further, it is possible to implement a motor mode, an engine mode, a hybrid mode, and a regenerative braking mode, etc. by controlling the speed of the motor generators and it is possible to control the engine to be turned on/off, if needed, thereby decreasing fuel consumption. Furthermore, it is possible to increase efficiency of regenerative braking by minimizing use of a friction brake in braking.
In an input type power train for a hybrid electric vehicle using two motor generators, one of two motor generators is fixedly connected directly or through a decelerator to an output shaft. The power train has the best efficiency at a deceleration ratio that creates a mechanical point where the speed of the other motor generator, which is not directly connected to the output shaft, is zero. The efficiency is decreased when the deceleration ratio decreases or increases from the above level. The efficiency decrease when the deceleration ratio decreases is significantly larger than when the deceleration ratio increases. That is, as the deceleration ratio decreases under the mechanical point (as vehicle speed increases), the efficiency of the power train rapidly decreases.
On the other hand, in a compound split type power train for a hybrid electric vehicle using two motor generators, both motor generators are not directly connected to an output shaft or an input shaft, but separately connected to rotary components of a planetary gear set. According to this power train, one mechanical point in the two motor generators can be created at a deceleration ratio that is smaller than that of the input split type, such that the power train can ensure high efficiency of the power train in relatively high-speed driving, but is disadvantageous in efficiency at relatively low speed due to power recirculation.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.